step 1 - get yourself to a Command Line on Zynthian - either “ssh” to Z or access terminal in webconf. If you don’t know how research or come back and ask.
step 2 - get into an editor on the file. vi is available along with many others. If you don’t know how you can find a VI tutorial on the web or in a book. Or ask if you get stuck here.
step 3 - add the section indicated by @oscaracena above.
step 4 - Try it!